Higher CO2 levels cause ‘greening’ from fertilization effect

Rising atmospheric carbon dioxide levels has triggered “greening” in arid regions around the world due to a fertilization effect that has increased plant growth, reports a new study published in Geophysical Research Letters.

The study is based on mathematical modeling of the extent of the CO2 fertilization effect and analysis of satellite imagery in dry parts of North America, Australia’s outback, the Middle East, and Africa. Controlling for precipitation, air temperature, the amount of light, and land-use changes, Randall Donohue of Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) and colleagues found an eleven percent increase in foliage between 1982 and 2010, when atmospheric CO2 levels increased 14 percent.

“Lots of papers have shown an average increase in vegetation across the globe, and there is a lot of speculation about what’s causing that,” said Donohue in a statement. “Up until this point, they’ve linked the greening to fairly obvious climatic variables, such as a rise in temperature where it is normally cold or a rise in rainfall where it is normally dry. Lots of those papers speculated about the CO2 effect, but it has been very difficult to prove.”

The researchers say that the CO2 fertilization effect could favor a transition from grasslands to woodlands in forests in these regions, provided precipitation levels are sufficient to sustain trees.

The authors add that tropical forests aren’t likely to see increased greening because, unlike grasslands, their leaf cover is “already about as extensive as it can get”.

“Leaf cover in warm, wet places like tropical rainforests is already about as extensive as it can get and is unlikely to increase with higher CO­2 concentrations,” explained a statement from the American Geophysical Union, which publishes Geophysical Research Letters. “In warm, dry places, on the other hand, leaf cover is less complete, so plants there will make more leaves if they have enough water to do so.”